Conductance in Co/Al[subscript 2]O[subscript 3]/Si/Al[subscript 2]O[subscript 3] permalloy with asymmetrically doped barrier

We report on dependence of conductance and tunnelling magnetoresistance on bias voltage at different temperatures down to 2 K in Co|Al[subscript 2]O[subscript 3](10 Å)|Si(δ)|Al[subscript 2]O[subscript 3](2 Å)|Permalloy magnetic tunnel junctions. Complementary low frequency noise measurements are used to understand the conductance results. The obtained data indicate the breakdown of the Coulomb blockade for thickness of the asymmetric silicon layer exceeding 1.2 Å. The crossover in the conductance, the dependence of the tunneling magnetoresistance with the bias voltage and the noise below 80 K correspond to one monolayer coverage. Interestingly, the zero bias magnetoresistance remains nearly unaffected by the presence of the silicon layer. The proposed model uses Larkin-Matveev approximation of tunneling through a single impurity layer generalized to three-dimensional case and takes into account the variation of the barrier shape with the bias voltage. The main difference is the localization of all the impurity levels within a single atomic layer. In the high thickness case, up to 1.8 Å, we have introduced a phenomenological parameter, which reflects the number of single levels on the total density of silicon atoms.United States. Office of Naval Research (Grant No. N00014-06-1-0235)National Science Foundation (U.S.) (Grant No. DMR-0504158)Madrid (Spain : Region) (Grant No. P2009/MAT-1726)Spain. Ministerio de Ciencia e Innovación (MICINN) (Grant No. MAT2006-07196)Spain. Ministerio de Ciencia e Innovación (MICINN) (Grant No. MAT2009- 10139)Spain. Ministerio de Ciencia e Innovación (MICINN) (Consolider Grant No. CSD2007-00010)European Science Foundation. EUROCORES Programme (Grant No. 05-FONE-FP-010-SPINTRA)European Science Foundation. Arrays of Quantum Dots adn Josephson Junctions (AQDJJ) programmeFundação para a Ciência e a Tecnologia (FCT) (Grant PTDC/FIS/70843/2006)Poland. Ministry of Science and Higher Educatio